The present invention relates to an adaptive digital filter for simulating indeterminate transmission characteristics of a transmission system and preventing hand-free communication singing.
Adaptive digital filters have been used to prevent singing caused by acoustic coupling upon mobile duplex communication or hand-free duplex telephone communication with a microphone and a loudspeaker. A typical adaptive digital filter used for the above purpose has the following arrangement. An input analog signal is supplied from an input terminal to a transmission system including a loudspeaker and a microphone. The transmission characteristics such as amplitude and frequency characteristics of this transmission system change in accordance with different states of acoustic coupling between the loudspeaker and the microphone and become indeterminate. An output from the transmission system is converted by an analog-to-digital (to be referred to as an A/D hereinafter) converter to a digital signal. The digital signal is supplied to one input terminal of a subtracter. The input analog signal is also converted by another A/D converter to a digital signal which is supplied to a nonrecursive digital filter having transmission characteristics similar to those of the transmission system. An output signal from the digital filter is supplied to the other input terminal of the subtracter. The subtracter calculates a difference between the output from the transmission system and the output from the digital filter. The difference signal from the subtracter serves as a signal in which the singing in accordance with the transmission characteristics of the transmission system is suppressed. The transmission characteristics of the digital filter are determined in accordance with a coefficient signal supplied from a correction circuit.
In the digital filter having the arrangement described above, when the level of the input signal exceeds an allowable input level of the A/D converter connected to the input terminal of the digital filter, the coefficient signal from the correction circuit is not normally supplied, so that the transmission characteristics of the digital filter become entirely different from those of the transmission system, resulting in an operation failure.